A generic method to characterize molecular interactions with no assay development would substantially benefit both functional proteomics research and drug discovery;however, none of the available approaches offers the efficient detection and low reagent consumption needed for the large number of measurements. This proposal addresses developing a sensitive enthalpic assay tool to detect bimolecular interactions involving proteins, ligands, RNA, DNA and other molecules of interest without requiring any special assay development. In particular, the assay does not require development of molecule-specific inhibitors, probes, detectors, attachment of labels, immobilization on surfaces, or chemical modifications of the molecules. The assay is based on direct detection of the enthalpy released during interaction of the species of interest. The detector, a nanocalorimeter, will enable measurement of interactions for molecular concentrations in the low micro-molar range using sample volumes of 200 nl. The technical goal of this work is to develop a detector that can detect enthalpies in the nanocalorie range with an effective measurement time of six seconds. This represents nearly three orders of magnitude reduction in the required sample and measurement time compared with conventional microcalorimetry. We propose to develop an array of 96 detectors and use it to study key aspects of HIV infection and life cycle. In addition to detecting interactions the enthalpy array can be used to perform titrations to determine binding constants. The effort integrates several engineering disciplines with applied physics and biochemistry. We have developed the initial proof of concept through a multidisciplinary effort bridging microfabrication with several engineering disciplines (mechanical, electrical chemical and system) as well as applied physics. We are collaborating with several senior biochemists to ensure that the system meets their scientific needs.